Instead, having read the whole thread, the most convincing answer to me (for what it's worth) was the theory that they're taking a much more aggressive stand on supplementals.
For one thing, this would explain the huge difference in responses around here, responses from "I don't see any change. What's everyone talking about?" to "help! my site has been trashed!"
It makes sense too. The web is expanding, in part driven by rampant spamming practices. Every increase in the web is an increase in resource expenditure for the search engines.
They can't index it all. They don't want to index it all. so they're actively making decisions about what they will and won't index.
And, again to combat spam, they may have even disenfranchised supplemental pages (ie they no longer get to vote).
And here's a wild conjecture: That could have a knock-on effect, where pages that had a lot of backlinks from supplementals lose their backlinks, and therefore themselves become supplemental- causing pages that they link to to lose links and so on. If that were true, you would see massive upheaval in some parts, and in other parts blue skies, a light breeze, and plain sailing.
<edit:> I just thought of a possible sting in the tail of the "ranking by existing traffic" algorithm. That is, google could look at a site and say "if we didn't exist, how much traffic would this site get?" and if the answer is "none" then down into the bottomless pit you go...